The validity areas in Young's modulus models of composite materials using finite element analysis (FEA) were identified. The bounds that these models hold on i.e., the proportion of phases, the modulus and Poisson's ratio of matrix and reinforcement phases, the geometry of the system, the loading conditions, the secondary effects etc., were accurately defined. Also, all critical factors of the respective models were computed and modified to describe and best approximate the "referring situation," which was chosen to be that of FEA. In addition, applying these models to a real composite and through the appreciation of the diversions in the reference state, it is possible to determine qualitatively the total behavior of a composite material and to estimate a range of real material features such as the true intensity of dispersedphase interaction, the appeared imperfections and aggregates, the grade of adhesion, the existence of mesophase, etc. Finally, from the incoming information by this analysis and by suitably forming and structuring these models, we raise up their simplicity and their uniqueness and so it was made possible to construct a modulus map for the limitation and applying areas of all modulus models. POLYM. COMPOS., 28:593-604, 2007. POLYMER COMPOSITES-2007 FIG. 2. The variation of modulus E c via percentage f as results from FEA and phase rule. (a) E f /E m ϭ 2 and 5 and (b) E f /E m ϭ 13 and ϱ. FIG. 3. The variation of modulus E c of composite via percentage f as results from finite element analysis and from Takayanagi-Kawai relation.